The present invention relates to a screen-printable paste which contains 40 to 75% by weight of an electrically conductive metal powder namely silver, aluminum, or copper and mixtures thereof, 5 to 20% by weight of an organic synthetic resin and 5 to 55% by weight of an organic solvent for producing electrically conductive coatings on flexible plastic sheets [sheetings, foils, films], especially for manufacturing membrane keyboards.
In order to produce thin, electrically conductive circuits on flexible substrates such as e.g. polyester sheets, screen-printable conductive pastes are required which must also be flexible after drying. The main area of application for these pastes are membrane keyboards, which were introduced into the market a few years ago and have gained increasing acceptance in the marketplace as a result of technical improvements. Membrane keyboards are presently used primarily in consumer electronics as well as in the operating elements of machines and other technical apparatus.
The basic design for membrane keyboards is described in DIN 42 115 (draft). According to it, the main component is a membrane keyboard display consisting of many individual membrane switch elements. Membrane keyboards and therewith also the membrane switch elements are generally constructed out of 3 layers.
The first layer consists of a flexible base sheet (usually polyethylene terephthalate) which is printed with a conductive paste using the screen-printing method and is subsequently dried at temperatures under 200.degree. C. A so-called spacer sheet is applied thereupon (adhesive on both sides) which comprises apertures at the contact points. The third layer is again usually a polyester sheet which is printed on the inner side with a conductive paste using the screen-printing method and subsequently dried. A contact between the conductive circuits of the first and of the third layer is created by means of external pressure on the uppermost (third) layer at especially designated positions (spacer apertures) in corresponding screen-print patterns. If a voltage is placed at the same time on two conductive circuit planes, a short-circuit current then flows which releases a control impulse.
Membrane keyboards exist in many variations. They can basically be divided into systems without and with pressure point (so-called dome). As a result of the increasingly greater demand for tactile response (feedback) in membrane keyboards, development is headed in the direction of variants with a stamped [impressed] dome. This involves a hot deformation of the sheet with the applied conductive circuits which places very high requirements on the flexibility, long-term adhesion and fatigue strength under reversed bending stresses of the conductive tracks.
In the prior art, U.S. Pat. No. 4,425,263 and EP-PS No. 0,068,168 describe screen-printable, electrically conductive pastes consisting of a conductive metal powder, an organic binder component and an organic solvent whose organic binder component consists of an aromatic polyester resin. Certain sheet resistivity values are achieved which, however, rise drastically after creasing or bending of the printed conductive circuits since the conductive circuits produced with these pastes are not sufficiently flexible and break.
U.S. Pat. No. 4,592,961 teaches an improvement of the flexibility of silver conductive circuits if a vinylidene copolymer is added to the aromatic polyester lacquer resin. However, such pastes gel quite rapidly and are then no longer screen-printable.
The use of epoxy resins for producing conductive masses and conductive pastes is known. Thus, U.S. Pat. No. 3,746,662 describes a conductive silver epoxy-resin adhesive which is used to fasten components to substrate materials. This adhesive consists of a low-molecular, liquid epoxy resin and contains an additional hardener component. It is not suitable for membrane keyboards due to its brittleness in the hardened state.